1. Field of the Invention
The present disclosure is generally related to radar systems and more specifically to correcting errors in Synthetic Aperture Radar (“SAR”) data.
2. Related Art
Radar has long been used for military and non-military purposes in a wide variety of applications such as imaging, guidance, remote sensing and global positioning. A Synthetic Aperture Radar (“SAR”) is a coherent mostly airborne or spaceborne side-looking radar system (“SLAR”) which utilizes the flight path of a moving platform (i.e., a vehicle such as, for example an aircraft or satellite), on which the SAR is located, to simulate an extremely large antenna or aperture electronically, and that generates high-resolution remote sensing imagery. Specifically, a SAR is used for terrain mapping and/or remote sensing using a relatively small antenna installed on the moving platform in the air.
The fact that a SAR may utilize a small antenna is a major advantage over a SLAR because the beamwidth of the radiation pattern of an antenna (generally known as the “antenna beamwidth”) is inversely proportional to the dimension of antenna aperture and, in general, the more narrow the antenna beamwidth, the higher the potential resolution of a scanned image of a target area. As such, the image of the target area formed by a SLAR is poor in azimuth resolution because the azimuth antenna beamwidth has an angular resolution that is too broad to differentiate between fine details at the same range within the target area. Therefore for an SLAR, either an extremely large antenna or very high frequency of operation (where the wavelengths are so short that the radar must content with the effects of severe attenuation in the atmosphere) needs to be utilized to produce high resolution images of target areas for direct terrain mapping and/or remote sensing, which is unrealistic for a radar system installed on a moving platform.
SAR solves the problems associated with SLAR by utilizing a smaller antenna and signal processing. Unlike a SLAR, a SAR is able to achieve the same effect of a large antenna by transmitting an array of pulses using a small aperture antenna and coherently processing the resulting data reflected from the terrain for all transmitted pulses. In general, SAR is a technique that utilizes signal processing to improve the resolution beyond the limitation of the physical antenna aperture on the platform where the forward motion of actual antenna is used to ‘synthesize’ a very long antenna. As such, SAR allows the possibility of using longer wavelengths and still achieving good resolution with antennas that have structures of reasonable size.
In an example of operation, the SAR moves with the platform along a flight path over a target and transmits successive pulses of radio waves to illuminate a target scene, receives and records the echo (i.e., the radar return signal) of each pulse, and then generates a high-resolution image of the target scene from processing the received echo pulses. In this way, the SAR works similar to a phased array system, but contrary of a large number of the parallel antenna elements of a phased array, the SAR uses one antenna in time-multiplex. As such, the different geometric positions of the antenna of the SAR, as a result of moving the platform, simulate the antenna elements of a phased array.
The SAR transmits the pulses at pulse repetition frequency (“PRF”) rate, which represents the number of pulses that are transmitted by the SAR per second. The reciprocal of PRF is also known as the pulse collection period or inter-pulse period. Each pulse is radiated (i.e., transmitted) at the carrier frequency of operation of the SAR during a transmit time (generally referred to as the pulse with (“PW”)). The SAR then waits for the returning echoes (i.e., the radar return signals) during a listening, or rest, time and then radiates the next pulse. The time between each transmitted pulse is known as the pulse repetition time (“PRT”) that represents the time between the beginning of one pulse and the start of the next pulse.
Over time, individual transmit and receive cycles (having a period of operation equal to the PRT) of pulses are completed by the SAR with the data from each cycle being stored electronically by a processor within the SAR. The data includes all the radar returned signals, as amplitude and phase values, for a time period “T” from a first position to second position of the SAR along the flight path. At this point, it is possible to reconstruct a radar return signal that would have been obtained by an antenna of length v·T, where “v” is the platform speed along the flight path.
The SAR then preforms signal processing on the stored data. The signal processing utilizes the magnitudes and phases of the received radar return signals over successive pulses from elements of a synthetic aperture. After a given number of cycles, the stored data is recombined (taking into account the Doppler effects inherent in the different transmitter to target geometry in each succeeding cycle) to create a high resolution image of the terrain being over flown by the SAR.
It is noted that as the line of sight direction changes along the flight path of the platform of the SAR, a synthetic aperture is produced by signal processing, where the signal processing has the effect of artificially lengthening the antenna of the SAR. As such, making T large makes the “synthetic aperture” of the SAR antenna large and hence a higher resolution of the SAR may be achieved.
In general, the SAR images produced by a SAR are two-dimensional images that consist of range and cross-range (i.e., the azimuth) direction values. It is appreciated by those or ordinary skill in the art that the azimuth resolution is inversely proportional to the collection period T during which targets are illuminated by the antenna beam. Therefore, a fine resolution in azimuth may be achieved by increasing the array time for a spotlight mode or by reducing the antenna aperture size for stripmap mode. Additionally, the range resolution is inversely proportional to the bandwidth of the transmitted signal. As such, a fine resolution in range may be achieved by increasing the bandwidth of transmitted signal.
Unfortunately, although it is true that in theory the range resolution may be improved by utilizing a wideband signal, it is also true that, in practice, increasing the bandwidth of transmitted signal beyond certain point is costly and difficult to physically implement in reality. This is because typically front-end hardware components in a radar system includes filters, amplifiers, and an antenna—all of which generally have degraded performance as the signal bandwidth of operation increases.
One approach to avoid this problem has been to divide the full desired wideband signal into a sequence of multiple narrow sub-band signals and transmit the sub-band signals in consecutive sub-pulses with stepped center frequencies. Then, the received signals reflected from any backscatters, for each transmitted pulse, are combined to produce a composite received signal that is equivalent to a received signal from a wideband signal that had been transmitted.
In SAR systems the most commonly utilized waveform signals are linear frequency modulation (“LFM”) signals. LFM signals are commonly referred to as “chirp modulation signals.” They employ sinusoidal waveforms whose instantaneous frequency increases or decreases linearly over time. It is appreciated by those of ordinary skill in the art that these sinusoidal waveforms have advantages over other types of waveforms and are commonly referred to as “linear chirps” or simply “chirps.”
Specifically, in a mode called “step chirp” or “stepped-chirp,” a stepped-chirp waveform is utilized to improve the range resolution of an existing pulse compression radar such as a SAR. The pulse is frequency modulated so as to help resolve targets which may have overlapping returns and where a desired full wideband signal is divided into multiple narrow-band sub-bands with their center frequencies stepped between them. The sub-band signals are sequentially transmitted in sub-pulses. Then, the signals reflected from backscatters on ground, from each transmitted sub-pulse, are received and combined to synthesize a wideband composite received signal that is utilized to produce a high resolution SAR image. Generally, this technique is suitable for obtaining high range resolution in a radar system that has a limited instantaneous bandwidth, but a large tunable bandwidth.
Although SAR images of high range resolution may be obtained by utilizing a step chirp process, there is still a need to avoid degradation of the image quality due to amplitude and phase errors in the composite signal produced from all the sub-pulses. Both amplitude and phase errors in the composite signal may include periodic components which are common to all steps and non-periodic components which vary between steps. Additionally, amplitude and phase discontinuities may occur at the step boundaries of a stepped-chirp waveform.
These errors are a problem in step chirp that can degrade formed SAR image if not properly estimated and corrected. In particular, periodic errors may cause undesirable paired echoes in formed image. Additionally, the non-periodic errors and amplitude and phase discontinuities also may cause image degradation in sidelobe area of the impulse response. Therefore, there is a need to estimate and correct these amplitude and phase errors.
Attempts to solve this problem in the past include a method for estimating the amplitude and phase error in single-step signal as described in U.S. Pat. No. 7,999,724, titled “Estimation and Correction Of Error In Synthetic Aperture Radar,” which issued Aug. 16, 2011 to inventor Kwang M. Cho and is herein incorporated by reference in its entirety. Unfortunately, this reference is limited in its disclosure to a single-step SAR and does not describe any way of correcting for errors in a step-chirp SAR.
As such, there continues to be a need in the art for a system and method to estimate and correct these amplitude and phase errors in a step chirp SAR system.